ABSTRACT Electronic cigarette (e-cigarette) use has become as an emerging public health concern. Millions of American people use e-cigarettes. Our previous study found that 15.3% of contemporary US adults ever used e-cigarettes, and 3.2% were current users of e-cigarettes. Despite their popularity, public health effects of e-cigarettes use are uncertain. E-cigarettes may aid in smoking cessation in some individuals, but they can cause harm to others. E-cigarettes usually contain nicotine and other toxic substances, such as heavy metals, propylene glycol, volatile organic chemicals, and carcinogenic agents. Pregnancy represents a unique period of many physiologic challenges, making it a time of vulnerability to other stressors, and thus a time of particular concern regarding potential effects of e- cigarettes. Our recent study showed that compared with non-pregnant women of reproductive age, pregnant women had a lower rate of conventional cigarette smoking but almost identical rate of e- cigarette use. Evidence from animal studies has shown detrimental effects of e-cigarette use in pregnancy. However, so far, no human studies are currently available that directly assess the impact of e-cigarette use on the health of pregnancy. As a recent Consensus Study Report from the National Academies of Sciences, Engineering, and Medicine concluded, ?there is no available evidence whether or not e-cigarettes affect pregnancy outcomes.? Clearly, there is an urgent and critical need for human studies to assess health effects of e-cigarettes on the mother and the fetus. The objective of this project is to determine the potential health effects of electronic cigarette use before and during pregnancy on pregnancy complications and adverse birth outcomes. We propose to conduct a large epidemiological study based on nationwide data from the Pregnancy Risk Assessment Monitoring System (PRAMS) in 2016. We will include 33,613 mothers who were aged 18 years or older and had a singleton delivery. The PRAMS participants reported whether, and how often, they used e-cigarettes during 3 months before pregnancy and in the last 3 months during pregnancy. The outcome variables include pregnancy complications (i.e., gestational diabetes, hypertensive disorders in pregnancy) and adverse birth outcomes (e.g., low birth weight, and preterm birth). Pregnancy complications are ascertained from both birth certificate and PRAMS questionnaire. Birth outcomes are ascertained from birth certificate. This large epidemiological study will, for the first time, address the effects of e-cigarette use on pregnancy and birth outcomes in humans. Successful completion of this project will provide important data for subsequent follow-up projects with R01 funding to fully understand the public health consequences of e-cigarettes during and beyond pregnancy. Eventually, these findings will inform a more balanced future public health policy regarding e-cigarettes.